1. Field of the Invention
The present invention relates to a microwave oven having a microwave sensor suitable for detecting a heating state or finishing state of a heated object the microwave oven.
2. Description of the Related Art
A microwave oven provides various functions such as thawing frozen food, warming chilled food, and the like by means of microwave heating. A microwave oven automatically controls the output of its magnetron to generate microwave energy by detecting with a sensor a heating state or a finishing state of such food.
A microwave oven which traces variation of temperature of the heated food from its frozen state to a thawed state and detects the end of a thawing cycle has been disclosed (Unexamined Published Japanese Patent application No. 64-50385). This microwave oven is equipped with a detector generating heat by absorbing microwave energy, a device to measure the temperature, and a computing and controlling device to control operation of the microwave oven from the temperature. The detector is located near an object to be processed in the microwave oven, and the computing and controlling device utilizes a curve showing temperature rise of the detector as a function of time, determines the end of a thawing cycle of the object by computing the value of a quadratic derivative (i.e., a derivative of the second degree) of this curve, and controls operation of the microwave oven at the end of the thawing cycle in which the value of the derivative of the second order is less than a specified value.
Another microwave oven equipped with a detector which can detect at a certain sensitivity the end of each thawing operation in a plurality of sequential thawing operations has also been disclosed (Unexamined Published Japanese Patent Application No. 64-50384). This microwave oven is also equipped with a microwave detector, a temperature measuring device, and a computing and controlling device. The detector of this microwave oven has a heat insulator which transmits microwave energy but prevents heat of the detector, which has been generated by absorbing microwave energy, from radiating outside. Since the heat insulator increases the temperature rise of the detector through reduction of heat exchange with the outside environment, the detector can monitor and detect each thawing operation without lowering its sensitivity. The heat exchange area of this detector is wide and is thin in thickness. This facilitates exchanging of heat by the detector with the outside environment and brings about a short thermal lag characteristic so as to quickly recover the initial characteristics after each thawing operation.
In the microwave oven described in Unexamined Published Japanese Patent Application NO. 64-50385, as an object proceeds from the icy state to the watery state, it is gradually heated by gradually absorbing more and more microwave energy, and the power absorbed by the detector gradually decreases.
When the slope (the linear derivative, i.e., the derivative of the first degree) of the curve representing temperature rise of the detector as a function of time is measured and the slope decreases to some degree and the absolute value of the quadratic derivative of the curve becomes greater than a specified value, the object in the microwave oven begins to thaw. When this slope becomes shallow and the absolute value of the derivative of the second degree of the curve becomes less than the specified value, the object has finished thawing. The above-mentioned microwave oven determines the thawing state from such variation of the quadratic derivative.
According to this method of determining a thawing state, however, it must be only the variation of the microwave power absorbed by the detector that causes the derivative of the second degree to change.
In general, t hours after a heated object, for example, a microwave sensor having heat capacity C has been receiving microwave power P, its temperature rise value .THETA. is represented by the following expression (2) in a completely adiabatic state in which no heat radiates outside at all. This relation is shown in FIG. 26. EQU .THETA.=P.multidot.t/c (2)
In an actual heated object, however, heat radiated outside cannot be ignored when it receives microwave power. If the heated object has a heat radiation constant .delta., the work P.multidot.dt which the object receives for a very short time dt is represented by the following expression (3). EQU P.multidot.dt=C.multidot.d.THETA.+.delta..multidot..THETA..multidot.dt(3)
where d.THETA. is the temperature rise of the object during a very short time, C.multidot.d.THETA. is a heat energy stored in the object during a very short time, and .delta..multidot..THETA. dt is a heat energy radiated outside during a very short time. From the above-mentioned expression (3), the temperature rise value .THETA. of the object is represented by the following expression (4) when the electric power P is constant. This relation is shown in FIG. 27. EQU .THETA.=(P/.delta.).multidot.[1-exp(-t/.tau.)] (4)
In this expression, .tau. is a thermal time constant ant has a relation of C=.tau..multidot..delta.. As seen in FIGS. 26 and 27, the difference grows between the temperature rise rates in the two cases as the temperature rise value .THETA. becomes larger.
Finding the linear derivative (d.THETA./dt) and the quadratic derivative (d.sup.2 .THETA./dt.sup.2) described in Unexamined Published Japanese Patent Application No. 64-50385 from the above-mentioned expression (4) results in the following expressions (5) and (6), respectively. These relations are shown in FIGS. 28 and 29. EQU d.THETA./dt=(P/.delta./.tau.).multidot.exp(-t/.tau.) (5) EQU d.sup.2 .THETA./dt.sup.2 =(-P/.delta./.tau..sup.2).multidot.exp(-t/.tau.)(6)
FIG. 29 and expression (6) show that the quadratic derivative (d.sup.2 .THETA./dt.sup.2) varies from (-P/.delta./.tau..sup.2) to 0 in a range of time from zero to infinity (0-.infin.) and is caused to change by heat radiation even when-the electric power does not vary with time.
This suggests that the method of determining a thawing state in the microwave oven described in Unexamined Published Japanese Patent Application No. 64-50385 is not accurate in a state in which the value e of temperature rise has increased. That is, the above-mentioned microwave oven determines the thawing state referring to variation of the quadratic derivative only from the variation of the microwave power absorbed by the detector, however, actually it is necessary to consider heat radiation of the microwave sensor.
The microwave oven shown in Unexamined Published Japanese Patent Application No. 64-50384 as described above, uses a heat insulator and adopts structure easy to radiate heat. As a result, (1) the heat insulator reduces heat radiation when microwave energy is irradiated, and (2) when microwave energy is not applied, the structure, which easily radiates heat, increases heat radiation and quickly returns the detector to the initial state, and further, prevents heat destruction caused by heat accumulation in the case of repeated heating.
However, the above-mentioned factors (1) and (2) are contradictory to each other, and it is impossible to full satisfy each of them.
In the case of considering heat radiation outside of the detector, the heat energy to be radiated depends on the ambient temperature. That is, when the ambient temperature is high, less heat energy is radiated, and when it is low, greater heat energy is radiated. For example, according to a condition of using a microwave oven, detection error becomes great when the temperature of the heating chamber is high. Since a single detector according to the prior art uniformly absorbs the ambient temperature, the microwave oven has been unable to accurately detect microwave power.